The invention relates to a process for production of directly reduced desulphurised iron.
The production of directly reduced iron takes place in a direct reduction process by the reduction of iron oxide with solid or gaseous reducing agents. Coal, for example, which reacts with carbon dioxide at higher temperatures and forms the reducing gas CO, serves as a solid reducing agent. As the coal contains a relatively large amount of sulphur and the iron ore used often also contains this element, both the iron produced and the waste gases have an excessively high sulphur content. Subsequent desulphurisation of the waste gases and the directly reduced iron is expensive.
Task of the Invention
The task of the invention is to propose a process for production of directly reduced, desulphurised iron.
General Description of the Invention
According to the invention this problem is solved by a process for production of directly reduced, desulphurised iron in a multiple-hearth furnace, which has two zones one above the other, each with several hearths, iron ore reacting with a carbon carrier at a temperature between 800xc2x0 C. and 1100xc2x0 C. and becoming metallic iron and the gases being desulphurised with desulphurising agents in a first zone of the multiple-hearth furnace, the directly reduced iron being discharged from the multiple-hearth furnace and the desulphurised gases conducted into the second zone, where they preheat the iron ore to between 600xc2x0 C. and 800xc2x0 C.
The sulphur content of the gases is reduced within the multiple-hearth furnace by this process. With direct reduction of iron ore sulphur is released during gasification of the reducing agent, e.g. coal. A smaller quantity of sulphur may also be released during the reduction from iron ore to iron. This sulphur is bound in this process within the first zone and can therefore no longer react, or react only to a lesser extent, with the iron ore and the iron formed. Hence the directly reduced iron produced according to this process has a clearly lower sulphur content.
The desulphurising agents contain, for example, lime (CaO), limestone (CaCO3) and/or magnesite (MgO). The desulphurisation of the gases advantageously takes place largely within this first zone by chemical reaction of the gases containing sulphur with these desulphurising agents, the sulphur reacting with the calcium and/or magnesium to form sulphates, sulphites, sulphides and the like. These compounds are formed preferably on the surface of the desulphurising agent. An advantage of these agents is that they have a positive effect on the slag properties when they are melted together with the directly reduced iron.
The multiple-hearth furnace has a first zone and a second zone, which each comprise several hearths. The second zone advantageously lies above the first zone and the gases rise from the first zone to the second one, whereas the solids are gradually transferred from the second zone into the first zone.
The solids, i.e. the iron ore and reducing agent, can be introduced separately or together into the multiple-hearth furnace. According to a preferred embodiment the iron ore is first charged to the top hearth of the multiple-hearth furnace in the second zone. It is circulated there by rakes, which project over the hearths, and gradually transferred to the underlying hearths. The reducing agent is then charged to a hearth further down in the multiple-hearth furnace, preferably in the lower section of the second zone, and mixed with the preheated iron ore. Agglomeration of the reducing agent and the ore is prevented by the continuous circulation. The reducing agents are, for example, bituminous coal, lignite, coke, etc.
According to a preferred embodiment desulphurising agents and a reducing agent are mixed with the iron ore in the second zone.
The desulphurising agents can be introduced together with the iron ore and/or the carbon carrier into the multiple-hearth furnace. However, they can also be introduced separately into the multiple-hearth furnace.
Depending on the sulphur content of the reducing agent the desulphurising agents can either be fed at one point into the multiple-hearth furnace or distributed and charged at various points. In the latter alternative, different particle sizes can also be used. It has proved to be advantageous to charge coarse-grained desulphurising agents in the second zone and powdered desulphurising agents in the first zone. Depending on the granulometry of the particles the latter are discharged with the gases or solids from the multiple-hearth furnace. Powdered desulphurising agent is advantageously introduced directly into the gas flow in the first zone, with the result that the desulphurisation is more effective. The powdered desulphurising agents enriched with sulphur are largely discharged with the gases from the multiple-hearth furnace.
The solids, i.e. the iron formed, the residues of the carbon carrier and the desulphurising agents are discharged from the multiple-hearth furnace and subsequently melted. The desulphurising agents together with the gangue of the ore form a slag, and the combined sulphur is disposed of together with the slag.
The reduction gases in the furnace can be adjusted to an optimum concentration by selective feeding of reducing agents into the lower hearths of the multiple-hearth furnace and thus a better degree of metallisation achieved.
In addition a gaseous reducing agent can be injected into the bottom hearths of the multiple-hearth furnace. Consequently more complete reduction of the ore is achieved.
All rising gases, including the volatile constituents of the reducing agents, can be subsequently burnt in the upper part of the furnace, i.e. in the second zone, and the residual heat of the gases inside the furnace can thus be utilised in an optimum manner. Good thermal efficiency is accordingly achieved, because the energy yield is better.
The multiple-hearth furnace can be operated under a specific overpressure to achieve a further increase in productivity.
According to a preferred embodiment the ore in the topmost hearths of the furnace is dried and preheated by the hot gases in the furnace and in contact with the hearths before it comes into contact with the reducing agent. The ore is heated to a temperature of at least 400xc2x0 C., preferably to at least 600xc2x0 C. to 700xc2x0 C., before the solid reducing agent is added.
Gases containing oxygen can be injected selectively on the hearth, where the heat requirement must be covered by combustion of the excess process gases, e.g. above the hearth, where the solid reducing agents are introduced.
It is advantageous to use gases containing oxygen which have a temperature of at least 350xc2x0 C.
In addition one or more hearths in the furnace which are below the hearth to which the solid reducing agents are introduced can be heated by burners.